Plasma Density Np Research Articles

Numerical study of the plasma meniscus shape and beam optics in RF negative ion sources

In order to extract intense ion beams with good beam optics from hydrogen negative ion sources, it is important to control the shape of the plasma meniscus (i.e. beam emission surface). Recently, it is suggested experimentally that the degradation of beam optics in the RF negative ion sources may be due to the fluctuation of the distance d eff between the meniscus and the extraction grid caused by the fluctuation of the plasma density np . The purpose of this study is to make clear the dependence of d eff on np in the presence of a large amount of surface produced H- ions in order to understand such fluctuation of beam optics in RF sources For the purpose, 3D electrostatic PIC simulation was conducted taking the bulk plasma density as a parameter, investigating the extraction region of a H- ion source. A large amount of the surface H- production on the PG has been taken into account under the assumption that the H- production rate is proportional to the bulk plasma density. The result shows that the effective distance d eff is proportional to np -1/2 even for a large amount of surface H- production. This dependence suggests that the bulk plasma density np is the key parameters to control d eff and the resultant beam optics extracted from the negative ion source.

Observation of the standing wave effect in large-area, very-high-frequency capacitively coupled plasmas by using a fiber Bragg grating sensor

The large-area capacitive discharges driven at very high frequencies have been attracting much attention due to their wide applications in material etching and thin film deposition. However, in the regime, the standing wave effect (SWE) becomes a major limitation for plasma material processing uniformity. In this work, a fiber Bragg grating sensor was utilized for the observation of the SWE in a large-area capacitive discharge reactor by measuring the radial distribution of the neutral gas temperature Tg. The influences of the RF power and the working pressure on the radial profiles of Tg were studied. At a higher frequency (100 MHz) and a lower pressure (5 Pa), Tg presents a center-peaked radial distribution, indicating a significant SWE. As the RF power increases, the central peak of Tg becomes more evident due to the enhanced SWE. By contrast, at 100 MHz and a higher pressure (40 Pa), the radial distribution of Tg shows an evident peak at the electrode edge and Tg decays dramatically towards the discharge center because the electromagnetic waves are strongly damped as they are propagating from the edge to the center. At a lower frequency (27 MHz), only edge-high profiles of Tg are observed for various pressures. For the sake of a comparison, a hairpin resonance probe was used to measure the radial distributions of the plasma density np under the same condition. The radial profiles of Tg are found to generally resemble those of np under various conditions. Based on the experimental results, the neutral gas heating mechanism was analyzed.

Effects of secondary electron emission on plasma density and electron excitation dynamics in dual-frequency asymmetric capacitively coupled argon plasmas

Effects of secondary electron emission (SEE) on the plasma density and electron excitation dynamics in dual-frequency (2 MHz and 14 MHz) capacitively coupled Ar plasmas are investigated. The plasma density np is measured with a hairpin probe, and the spatio-temporal distribution of electron excitation rate (ground state into Ar 2p1 state) is determined by phase resolved optical emission spectroscopy. It is generally found that as the low-frequency (LF) voltage ϕL increases, np first decreases at low ϕL, due to compressed the plasma bulk length by the LF source, and then increases slightly with ϕL, suggesting that the plasma is dominated by the α mode. When ϕL exceeds some critical value, np increases dramatically with ϕL, due to significantly enhanced ionization by secondary electrons, indicating a α–γ mode transition. An excitation pattern caused by SEE at the edge of the completely expanded LF sheath is observed at relatively high ϕL. Under various conditions, including the high-frequency voltage ϕH, the pressure p, the electrode gap L, and the electrode material, different dependences of np on ϕL are also discussed. It is found that the discharge turns into the γ mode at a lower ϕL when ϕH is higher. As p increases, the density peak moves axially towards the powered electrode, due to reduced sheath thickness, while its distance to the powered electrode is almost independent of other external conditions (ϕH, L, electrode material, etc). A higher p or higher L is favorable for the enhancement of np in γ mode, because secondary electrons can contribute more to the ionization. Due to the higher SEE coefficient of the aluminum electrode, a more significant increase in np in the γ mode can be seen than that with a stainless steel or copper electrode. Meanwhile, the spatio-temporal distributions of the electron excitation rate under the same conditions are analyzed to further understand the SEE effects.

Correlation between vibrational temperature of N2 and plasma parameters in inductively coupled Ar/N2 plasmas

Vibrational temperature (Tvib) of N2 gas and electron energy distribution function (EEDF) were measured in Ar/N2 mixture inductively coupled plasma (ICP). At a low gas pressure of 5 mTorr where the EEDF is bi-Maxwellian distribution, plasma density np and Tvib (from 7000 K to 5600 K) slightly decrease. However, remarkable decrease in np and Tvib is found with the dilution of N2 gas at a high gas pressure of 50 mTorr, where the EEDF is depleted Maxwellian distribution at a fixed ICP power of 150 W. When the ICP power increases from 150 W to 300 W at the gas pressure of 50 mTorr, the depleted tail on the EEDF is replenished, while np is little changed with the dilution of N2 gas. In this case, Tvib slightly decreases from 9500 K to 7600 K. These results indicate that the variation of Tvib is strongly correlated to the plasma parameters, such as the plasma density and EEDF.

Spatial distribution study of argon radio-frequency inductively coupled plasma in inverse hysteresis transition area

Parameters of radio-frequency (rf) inductively coupled plasma (ICP) have been investigated with Langmuir probe and optical emission spectrum (OES) in this paper. Through the observation results of OES, we can finally get that inverse hysteresis phenomenon would occur when the air pressure is 0.6 Pa. The mode of plasma is demonstrated based on OES while spatial distribution of electron temperature Te, plasma density np and plasma potential Vp at the same power and pressure in inverse hysteresis transition area could be assured with Langmuir probe. The experiment results indicate that even though Te, np and Vp distribute more homogeneously in both axial and radial direction at the stage of H mode than E mode, the result shows no great difference of the data in both H mode and E mode.

Betatron radiation damping in laser plasma acceleration

AbstractWe explore the feasibility of accelerating electron beams up to energies much beyond 1 TeV in a realistic scale and evolution of the beam qualities such as emittance and energy spread at the final beam energy on the order of 100 TeV, using the newly formulated coupled equations describing the beam dynamics and radiative damping of electrons. As an example, we present a design for a 100 TeV laser-plasma accelerator in the operating plasma density np = 1015 cm−3 and numerical solutions for evolution of the normalized emittance as well as their analytical solutions. We show that the betatron radiative damping causes very small normalized emittance that promises future applications for the high-energy frontier physics.

Production of high-density hydrogen plasmas by helicon waves in a simple torus

High-density plasma production by helicon waves in a simple torus has been investigated. The measured plasma density np versus external magnetic field B0 peaks at a condition close to the lower-hybrid resonance in H2, D2, and He. Three density regions are observed in the variation of rf power Prf. In the low-density region, helicon waves are not excited and plasmas are produced by an antenna induction field. In the medium- and high-density (H) regions, helicon waves with the m=+1 azimuthal mode are excited and plasmas are produced by the waves. In H2 plasmas, the second-radial mode is dominant over the first-radial mode at Prf>10 kW in the H region. The measured kz−np relation (kz is the wave number parallel to B0) of helicon waves in the H region shows deviation from that of the m=+1 mode derived using the uniform-plasma assumption. The kz−np relation is calculated for various radial-np profiles; parabolic, modified-parabolic, and Gaussian profiles. The difference in the kz−np relation of the m=+1 mode between the uniform- and nonuniform-plasma models is explained by adding a vacuum region between a uniform plasma and the radial boundary.

Contribution of slow waves on production of high-density plasmas by m=0 helicon waves

The contribution of the slow waves (SWs) on high-density plasma production by m=0 helicon waves is investigated in H2, D2, He, N2, Ne, Ar, and Xe plasmas. The density-jump is observed in all the gases used at optimum external magnetic fields B0. The measured plasma density np as a function of B0 peaks at a condition close to the lower-hybrid resonance in H2, D2, He, N2, and Ne, whereas, the measured dispersion relation indicates the excitation of helicon wave. Dispersion relation of electromagnetic waves is solved using dielectric tensor elements of a cold uniform plasma including ion terms, which shows two branches: the SW and fast (helicon) wave. The transition from the ionization by SWs, which are excited in the low-density plasma produced by an antenna induction field, to that by helicon waves has been predicted to explain the mechanism to produce the high-density plasma in the helicon wave discharges.

Characteristics of the high density plasma production by m=0 helicon wave

The characteristics of the high density plasma production by the m = 0 azimuthal mode helicon wave is investigated. By varying the power of the applied radio frequency Prf two discharge modes exist; one is the low density mode (plasma density np<1012 cm−3), at which the discharge is sustained not by a helicon wave but by the antenna induction field at low Prf; the other is the high density mode (np≳1013 cm−3), in which helicon wave is excited at high Prf; in between the two modes, np jumps more than an order of magnitude. The threshold Prf for the density jump is higher for the m = 0 mode than that of the m = +1 and −1 modes excited by the helical antenna.

A theoretical model of bulk plasma generated by the electron-cyclotron-resonance mechanism

A theoretical model is developed to explain properties of the electron-cyclotron-resonance (ECR) plasma and an experiment is conducted to compare the theoretical predictions with experimental measurements. A theoretical expression of the electron temperature Te is obtained assuming that the plasma electrons are heated by the transverse electric field of the input microwave. The electron temperature is expressed as Te=a+bp−1/2, where p is the plasma-chamber pressure, and the coefficients a and b should be determined from system parameters. Making use of the ambipolar diffusion, the plasma potential φ is described in terms of the local plasma density np. That is eφ(r,z)=Te ln[np(r,z)/nw], where −e is the electron charge and nw is the plasma density at the surface of the chamber. For the case of low neutral density, the plasma density np is also expressed as np2=γ2p exp(−εi/Te)−γ∥, where εi is the ionization energy of neutrals by electrons and the constants γ2 and γ∥ should be determined from system parameters. These theoretical predictions agree reasonably well with experimental data.

Importance of Magnetic Effects in a Two-Flow Model for Extragalactic Radio Jets

We propose a model for extragalactic radio jets in which two different flows of particles are taken into account, (i) a beam of relativistic electrons and positrons extracted from the funnel of accretion disc and responsible for the observed superluminal motion, (ii) a classical or mildly relativistic wind of electrons and protons coming out from all parts of the disc (Sol et al., 1989). Studying the mutual interaction of the two flows, we show that the configuration is not destroyed by the plasma-beam instability as long as the magnetic field, assumed longitudinal, is strong enough, with an electron gyrofrequency ωc = eB/mec greater than the ambient plasma frequency ωp = (4πnpe2)1/2 (Pelletier et al., 1988). When ωc < ωp, the relativistic beam loses its energy and its momentum mainly through the development of strong Langmuir turbulence in the wind, and disappears quietly after some relaxation zone where heating and entrainment of the wind occur. This emphasizes one aspect of the important role likely played by the magnetic field in the dynamics of extragalactic jets and provides one example in which the magnetic field, acting on the microscopic scale of an interaction, induces strong effects on large–scale structures. Detailed data on the closest known superluminal radio source 3C120 (Walker et al., 1987, 1988; Benson et al., 1988) allow a check on the likelihood of our model. Observational estimates of the variation along the jet of the magnetic field and of the ambient plasma density np suggest that the magnetic field reaches its critical value (corresponding to ωc = ωp) at a minimal distance of about 1.4 kpc from the central engine. This is amazingly close to the location of the 4′–radio knot, a “rather curious structure” described by Walker et al. (1987), which we interpret as the beam relaxation zone in the context of our two–flow model (Sol et al., 1989).

The effect of background plasma in the undulator on free electron lasers

A new unconventional FEL scheme (named PFEL), which has injected rarefied ionized background plasma in the undulator, is studied theoretically. It is shown that there exists a critical density np; for the background plasma. When the background plasma density np is higher than np c, the FEL can no longer be excited. When np ≲ np c, the laser gain for Raman FEL may be increased noticeably.